Technical Field
The present invention relates to a navigation device, a route search server, and a route search method, and relates specifically to a technology for performing route guidance that excludes a difficult-to-travel section appropriately and provides an alternative route with favorable route quality.
Related Art
There is known an automotive navigation system (hereinafter referred to simply as a navigation system) that guides a driver by displaying, on a screen, an estimated current location of a host vehicle on a road map, a route leading to a destination, and other information.
Such a navigation system typically sets a route by using digital map data (hereinafter referred to as map data) which handles roads as links and intersections as nodes, and calculating a route with the smallest sum of link costs, such as time, distance, and fuel consumption for a travel from the departure point to the destination. Such a function is known as a route search function, and Dijkstra's algorithm or similar algorithms are widely used to implement this route search function.
Information on a route found by a navigation system is used for guidance of a driver using screen-display and/or voice message, and is also expected to be used for control of an autonomous vehicle autonomously driving to the destination. Thus, what is important is the safely for the vehicle in travelling the actual roads by following the found route.
Meanwhile, a conventional navigation system sometimes finds a route that includes a section that is difficult to travel in an actual road (such a section is hereinafter referred to as a difficult-to-travel section). An example thereof is a route including a section where the vehicle should merge into the far left lane of a multilane road, make multiple lane changes to move to the far right lane while traveling a short distance to the next intersection, and then make a right turn at the next intersection. Such a route is not preferable since the safety may be impaired in terms of both the guidance of the driver and the control of the autonomous vehicle.
As a conventional technology for setting a guidance route without including a route that requires unsafe lane change (i.e., a difficult-to-travel route), there is proposed, for example, a route setting device comprising a route setting unit that sets a guidance route from a departure point to a destination based on link information on links which couple nodes and coupling information on coupling between the links, the route setting unit being capable of setting the guidance route without including a difficult-to-travel route which is a route including a coupling point to a link with two lanes in each direction and a right-turn, left-turn, or divergence point ahead of the coupling point, and having a calculation value not satisfying a predetermined determination condition, the calculation value obtained based at least on the distance from the coupling point to the right-turn, left-turn, or divergence point and on the number of lane changes required while travelling that distance (see Japanese Patent Application Publication No. 2001-330459).